1. Field of the Invention
The present invention relates to a rotary polygonal mirror driving apparatus used in laser printers as an optical deflecting device. More specifically, the present invention relates to a mounting structure of a rotary polygonal mirror to a rotor.
2. Description of the Related Art
A rotary polygonal mirror driving apparatus used in laser printers as an optical deflecting device, as disclosed in, for example, Tokkai H8-205452 comprises a rotor to which a driving magnet is attached, a stator disposed opposite the driving magnet, a rotary polygonal mirror having a center hole through which a portion of the rotor is inserted, and a mirror pressing member mounted to the rotor under the condition that the rotary polygonal mirror is pressed in the thrust direction.
In the rotary polygonal mirror configured in this manner, a thinly cut groove is provided along the chord of an annular top surface of a cylindrical portion (fitting protrusion) of the rotor which is fitted to the center hole (the fitting hole) of the rotary polygonal mirror, and the portion further out than the groove is made to be a elastically deforming portion. In the center of the groove on the top surface of the rotor, a screw hole is formed for screwing a screw that is used for fixing the mirror pressing member to the rotor. Therefore, even if the outer diameter of the fitting protrusion is small compared to the center hole of the rotary polygonal mirror, by screwing the mirror pressing engagement crew into the screw hole, the elastically deforming portion is widened outwardly, thus eliminating the gap between the elastically deforming portion and the inner circumferential surface of the center hole of the rotary polygonal mirror.
However, with the conventional mounting structure of the rotary polygonal mirror, if the force of tightening the screw is insufficient, a large centrifugal force is exerted on the rotary polygonal mirror rotating at high speed to expand it, widening the center hole and accordingly causing vibration in the rotary polygonal mirror. As a result, the rotary polygonal mirror is displaced on the rotor losing balance, and therefore, a great vibration is generated. On the other hand, if the screw (for widening the diameter) is tightened too much in order to firmly close the gap between the elastically deforming portion and the inner circumferential surface of the center hole of the rotary polygonal mirror even during the rotation, a large stress is applied to the inner circumferential surface of the center hole of the rotary polygonal mirror, possibly distorting a mirror surface formed on the outer circumference of the rotary polygonal mirror.
Considered may be a configuration in which the mirror pressing member elastically presses the rotary polygonal mirror in the thrust direction to fix it to the rotor, leaving the gap between the inner circumferential surface of the center hole of the rotary polygonal mirror and the outer circumferential surface of the rotor. With this configuration, there is no problem when the rotary polygonal mirror rotates at the speed of 10,000 turns per minute; however, when the polygonal mirror rotates at the high speed rotations of 37,000 per minute and repeats such high speed rotations and suspensions, the rotary polygonal mirror receives a large centrifugal force to expand and contract at the suspension. Consequently, a small displacement occurs in the rotary polygonal mirror at rotation, which cannot be found at the suspension, degrading optical polarizing performance.